This lesson begins section 5 of the foundation routine. “Cai” or “pluck” draws the opponent’s force away from their centre of rotation. It has the effect of uprooting the opponent and pulling them off balance, while at the same time improving your balance and increasing mechanical efficiency.
You can learn more about “pluck” and the 13 postures in the video “Introduction to Tai chi Principles, Episode 16: The thirteen postures.”